1. Field of the Invention
The present invention relates to a method for stretch blow molding a wide-mouthed container which is used as a packaging container for processed food, confectionery, or the like and which is made of thermoplastic resins.
2. Detailed Description of the Related Art
Stretch blow molded packaging containers which is made of thermoplastic resin and which have a thin wall thickness of a body portion are known. Examples of the thermoplastic resin used therefor includes polyesters, polypropylene and polycarbonate, and particularly bottles for beverages made of polyethylene terephthalate are generally and widely known as so-called xe2x80x9cPET bottlexe2x80x9d.
One of the stretch blow molding methods is a method in which a preform having a mouth portion, a body portion and a bottom portion is injection molded within an injection mold, the perform is released from the injection mold in a state where the inside thereof is still in a high temperature while holding the mouth portion of the preform, which is cooled and solidified, with a lip mold, the preform is transferred to a blow mold as it is, and the preform is stretch blow molded within the blow mold into a container such as a bottle having a thin wall of the body portion.
In such a molding method, the temperature of the outer and inner surfaces of the preform is lowered below an orientation temperature due to cooling during injection molding. After releasing the preform, the outer and inner surfaces of the preform are heated by utilizing the heat inside the preform till performing stretch blow molding to increase the surface temperature to the orientation temperature. At the same time, the temperature inside the preform body portion except the mouth portion held by the lip mold is lowered to the predetermined orientation temperature. After then, stretch blow molding is performed.
By the way, a wide-mouthed container used for a packaging container for jam, peanut butter, candies or the like has a wider mouth portion than a bottle for beverages. Further, because of the shape of the container, outside diameter of the body portion thereof is limited so as to be a slightly larger than that of the mouth portion to some extent. In case where the body portion of the preform for a wide-mouthed container is formed similarly to that for a bottles in a longitudinal direction, since the radial stretching ratio is small, it is difficult to obtain a container of which body portion is made thinner wall by air blowing. In order to solve this problem, a preform for a wide-mouthed container is formed to have a conical-shaped body portion or a truncated-conical-shaped one, thereby enabling of setting the stretching ratio relatively large.
When the preform for such a wide-mouthed container is stretch blow molded after lowering the preform temperature to a set temperature after releasing in a high temperature state, a connecting portion between the mouth portion and the body portion becomes thicker than other portions of the body portion. As a result, thinning of the body portion just below the mouth portion cannot be achieved, and in some cases a ring-shaped thicker portion appears just below the mouth portion. This makes the appearance of the resulted container worse as a packaging container, and it would be considered as a reject.
The reason why this connecting portion becomes thicker is that heat at the connecting portion is transferred to lip mold or blow core inserted into the mouth portion during stretch blowing, resulting in heat loss at the connecting portion. After releasing the preform, it is transferred to the blow mold while holding the mouth portion of the preform with the lip mold. Accordingly, during the transferring after releasing and after transferring the preform within the blow mold, heat exchange between the lip mold and the connecting portion of the body portion which is still in a high temperature state via the mouth portion is still continued. As a result, temperature at the connecting portion is lowered below the temperature of the other body portion which is exposed to the atmosphere, and it is difficult to stretch the connecting portion more than the other body portion.
In order to solve the problems, in the conventional method, the heat loss due to the heat exchange with the lip mold is compensated by increasing the thickness of the associated body portion to increase the heat amount to be held thereby. However, there is a certain limit to increase the wall thickness. For example, if the wall thickness of a preform made of polyethylene terephthalate is 4.0 mm or more, the outer surface where the wall thickness thereof is thickened is whitened during its transferring to the blow mold. This results in a light whitened stain on the container molded by stretch blow molding. Particularly, in such a case where the wide-mouthed container for packaging, as shown in FIG. 4, is formed to have a tapered surface at the upper body portion to be capable of visual inspection of contents from outside therethrough, such a defect would be a fatal damage and deteriorate commodity value thereof. Thus, there is a problem that such a container cannot be commercialized.
The present invention has been deviced to solve the above mentioned problems for the wide-mouthed container by stretch blow molding, and an object of the present invention is to provide a novel method for stretch blow molding a wide-mouthed container which is capable of stretching the entire body portion including the connecting portion with the mouth portion to a thinner wall thickness by employing a preliminary stretching within a blow mold before stretch blowing without making the upper body portion be thicker.
In order to achieve the above-mentioned object, the present invention is to provide a method for stretch blow molding a wide-mouthed container, comprising the steps of injection molding a preform having a large diameter mouth portion, a body portion and a bottom portion within an injection mold, releasing the perform from the injection mold in a state where the inside thereof is still in a high temperature while holding the mouth portion of the preform, which is cooled and solidified, with a lip mold, transferring the preform to a blow mold as it is, and stretch blow molding the preform into a desired container, wherein before the stretch blowing step of the preform a stretching rod is descended to a predetermined length to preliminary stretch the body portion in some extent, thereby thinning a connecting portion of the upper body portion between the body portion and the mouth portion, and after then the entire body portion is stretch blow molded.
Further, in the present invention, the body portion of the preform is formed in a conical shape in which the body portion is made thicker than the mouth portion and the diameter of the body portion is gradually decreased towards the bottom portion, and the body portion is preliminary stretched to about 10 mm length to thin the wall thickness of the upper body portion.
According to the present invention, the wall thickening phenomenon at the connecting portion between the mouth portion and the body portion of a wide-mouthed container can be improved, thereby enabling to obtain a wide-mouthed container for packaging which has a uniform body wall thickness and a well finished connecting portion. As to the molding step therefor, it is not required to use any additional special means or the like, but a preliminary stretching by a stretching rod in a vertical direction is simply carried out as an additional step before the stretch blow molding step. Accordingly, the present invention is directly applicable to the above-mentioned conventional molding method without increasing the molding cycle, thereby solving the problems as above.